It really depends on your computer specs - CPU speed, available RAM and so on. I've created scenarios with many actors on my main PC that runs fine, transferred it across to my laptop and it's run like a dog. There's no hard limit put in place by Greenfoot (save the memory in the config file that it's chosen to start with). Realistically speaking it pays to have a sensible amount of actors in the world, and / or test on a less powerful PC to make sure that others can run your game. Otherwise what might work fine for you might not play fine on many computers!